1. Field of the Invention
The present invention relates to a lubricating oil composition, in particular, a lubricating oil composition having improved friction reducing properties and suitable for use as a lubricating oil for internal combustion engines, automatic transmissions, suspensions and power steering wheels, particularly as a lubricating oil for internal combustion engines.
2. Description of the Related Art
Lubricating oils are usually used for smoothing the working of internal combustion engines, driving mechanisms such as automatic transmissions, suspensions and power stearings, and gears. Particularly, engine oils are effective in lubricating mainly sliding parts such as a piston ring and a cylinder liner, bearings of a crank shaft or a connecting rod, and valve train including cams and valve lifters; in cooling the engine; in cleaning and dispersing combustion products; and in preventing rust formation and corrosion.
Thus, various functions are required for the engine oils and, recently, even better functions are being demanded as the required performance and engine output become higher and higher and the operation conditions more severe. Under these circumstances, additives such as an antiwear agent, metallic detergent, ashless dispersant and antioxidant are incorporated into the engine oil in order to satisfy such requirements.
Since the energy loss in the friction parts in which the lubricating oil participates is high in the engine, the lubricating oil is used in combination with additives such as a friction modifier (FM) in order to minimize the friction loss and improve the fuel consumption (see, for example, Japanese Patent Publication No. 23595/1991). Further, since the engine oil for automobiles is used under the various conditions of oil temperature, engine speed and load, more excellent frictional properties under the wide-range conditions of use are necessitated for further improving the fuel consumption.
The present invention has been completed after investigations made for the purpose of providing a lubricating oil composition having improved friction reducing properties over those of an ordinary lubricating oil containing molybdenum dithiocarbamate and a phosphoric ester, and is suitable for use as a lubricating oil for internal combustion engines, automatic transmissions, suspensions and power steering wheels, particularly as a lubricating oil for internal combustion engines.